The present invention relates generally to electrostatic grounding equipment and particularly to body straps intended to be worn by individuals to prevent the accumulation of electrostatic changes.
Generally, a need exists for devices to control the electrostatic charge accumulation on the body or person of an individual. Certain individuals occupy areas or handle materials in which an electrostatic discharge could either be hazardous to the individual or could damage the material being handled. Examples are individuals in the proximity of an explosive or hazardous environment and indivduals who must handle electostatic sensitive electronic components.
Many devices have been developed to solve the problem of electrostatic charge accumulation and subsequent discharge. These include devices which have been fastened into body straps or wrist straps to be worn by the particular individual involved. These body straps or wrist straps are then adapted to be connected to a ground potential, possibly through a predetermined resistance, in order to dissipate the electrostatic charge accumulation on the individual and to prevent additional electrostatic charge accumulation. These devices operate by draining off any accumulated electrostatic charge from the individual.
One of these devices is the model 2064 wrist strap manufactured by Minnesota Mining and Manufacturing Company (3M), St. Paul, Minn. The model 2064 wrist strap consists of a Velostat (Velostat is a trademark of the 3M Company) strip held on the wrist with a band of nylon. Velostat conductive material is a carbonloaded conductive polymer. The operation of the wrist strap relies on the conductive polymer to conduct electrostatic charge via the individual's wrist to a ground cord secured to the wrist strap with an electrically conductive snap connection. The wrist strap relies on a hook and loop fastener system (e.g., Scotchmate, a trademark of 3M, fastener or Velcro, a trademark of Velcro U.S.A., Inc., Manchester, N.H., fastener) to secure the wrist strap to the wrist of the individual wearer.
A wrist strap manufactured by Semtronics Corporation, Peachtree City, Ga., is constructed from similar functional components. The Semtronics wrist strap also uses a black conductive plastic secured to the wrist with a hook and loop closure system.
A wrist strap manufactured by Simco, Landsdale, Pa., also uses a similar system. The entire band of the wrist strap is made of a nylon hook and loop fastener system. The Simco wrist strap has a carbonloaded conductive material secured to the inner surface of the hook and loop fastener. A snap connection is provided for a ground cord. The Simco wrist strap again relies on the conductive polymer for conducting the electrostatic accumulation on the individual to the snap connection and to the grounding cord. Similarly, the Simco wrist strap also relies on the hook and loop fastener for the closure system.
A wrist strap manufactured by Wescorp of Mountainview, Calif., consists of a carbonloaded conductive fabric with a hook and loop fastener. The Wescorp wrist strap relies on the conductive fabric for the conduction of electrostatic charge from the individual instead of the conductive polymer as in the previous straps but again relies on the hook and loop fastener for the closure system. The Wescorp strap also utilizes the conductive fabric for a connection to ground rather than a connection point to a ground wire.
A strap manufactured by Walter G. Legge Company, New York, N.Y., carrying the name "WRISTSTAT" uses a black nylon band with a hook and loop fastener. A conductive polymer is attached to the band with a metallic plate at a relatively narrow location around the strap. The conductive polymer also has a snap connection to a ground cord. The Legge wrist strap relies on the middle plate and the conductive polymer for conductivity and relies on the nylon band with the hook and loop fastener for the closure system.
Wescorp also has a strap consisting of a metallic bead chain to which an electrical ground cord is slidably attached. The strap relies on the metallic beads for conductivity. Since it is worn loosely around the wrist, it can be made large enough to slip over the hand onto the wrist and, thus, no detachable closure is required.
Controlled Static Company, Santa Fe, Calif., manufactures a wrist strap known in the trade as a Fred strap. The strap is a metallic expansion band having a snap connection for an electrical ground cord. The band is reminiscent of a metallic expansion watch band. The band relies on the conductivity of the metal for the drainage of the accumulated electrostatic charges and will expand to slip on the wrist over the hand and then fit relatively snuggly.
U.S. Pat. No. 4,398,277, Christiansen et al, assigned to 3M, discloses a conductive elastomeric fabric and body strap constructed from that fabric. In a preferred embodiment in Christiansen et al, a wrist strap is formed utilizing a knit fabric having electrically conductive fibers and elastomeric fibers as well as an insulative fiber. The knit band is constructed with the electrically conductive fibers on the inner surface, the insulative fibers on the outer surface and with the elastomeric fibers allowing for extensibility of the strap.
U.S. Pat. No. 4,577,256, Breidegam, assigned to Semtronics Corporation, Peachtree City, Ga., describes a woven stretchable grounding strap. The strap has conductive fibers on the inside surface of the strap to contact the skin and conduct electrical charges to a grounding cord attached to the strap. An adjustable mechanical clasp is provided to provide an adjustable sized wrist strap.
All of the above wrist straps provide a mechanism for draining accumulated electrostatic charges from the body of the wearer of the wrist strap. All of the wrist straps provide an electrically conductive surface in contact with the wrist, a mechanical closure system for closing the strap around the wrist and a point for the electrical connection of a ground cord. All of the above described wrist straps, however, are relatively expensive to construct since they all must fit around the wrist and require the wearer to place a strap around his wrist in addition to a watch band he may be currently wearing.
U.S. Pat. No. 3,546,800, Tate, Jr., discloses one of a variety of known calendar attachments for a wrist watch band. As described in Tate, Jr., such calendar attachments are formed from a flat plate of a soft flexible metal which can be manually wrapped around the wrist watch band and bent into place to clamp the band and having a central portion onto which is printed indicia representing a calendar. While such devices are effective in providing ready access to a calendar for the wearer of a watch band, the device has absolutely nothing to do with the electostatic charge buildup on the wearer.